Metal treatment – Stock – Ferrous
Patent
1994-12-22
1995-10-31
Yee, Deborah
Metal treatment
Stock
Ferrous
420 60, C22C 3820
Patent
active
054626110
DESCRIPTION:
BRIEF SUMMARY
FIELD OF APPLICATION IN INDUSTRY
The invention relates to a ferritic stainless steel excellent in high temperature oxidation resistance and scale adhesion suitable for use in the manufacture of exhaust gas tubular members in exhaust gas systems connected to various internal-combustion engines, in particular, in the manufacture of automotive exhaust gas manifolds.
BACKGROUND OF THE INVENTION
Because of recent growing concern on environmental pollution problems, there are eagerly desired thermal power generation systems or engines which can exhibit high combustion efficiency and automotive engines which can clear recent severe regulations on exhaust gases. Measures taken for these purposes result in an combustion gas having a higher temperature and, in consequence, adjacent members, e.g. a system for purifying an exhaust gas, connected to the thermal power generation engines or automotive engines are necessarily encountered to the higher temperature. Accordingly, further enhanced heat resistance is required for them. For heat resistance of a material, high temperature oxidation resistance, that is a property of the material that it is well durable under an atmosphere of a high temperature gas, is required, in addition to high temperature strength of the material.
In order that a material is excellent in high temperature oxidation resistance for a purpose of the invention, it is desirable that the material does not undergo abnormal oxidation and exhibits the smallest possible oxidation weight gain during its service. Besides, the material should be excellent in adhesion of oxide scale (oxide film). Since operation of internal-combustion engines such as automobile engines includes repeated starting and stopping of driving and thermal power generation systems are operated in daily start and stop (DSS) fashion, heat-resisting members connected to such engines of systems are subjected to repeated cycles of heating and cooling. Accordingly, with materials poor in adhesion of oxide film, it spalls posing problems of clogging of piping and reduction in thickness of the heat-resisting members themselves which might lead to mechanical breakage of the members.
Austenitic stainless steels have better high temperature strength than ferritic stainless steels. However, because of their larger thermal expansion and in turn larger thermal strain, when compared with ferritic stainless steels, austenitic stainless steels are liable to cause cracking due to thermal fatigue when subjected to repeated cycles of heating and cooling. Moreover, because of their large difference between thermal expansion of the base metal and that of oxide scale, austenitic stainless steels exhibit an increased amount of scale spalling.
For these reasons ferritic stainless steels have been used in the manufacture of automobile exhaust gas systems. For example, a ferritic stainless steel, SUS430JIL has been used in the manufacture of an automobile exhaust manifold. This steel, however, poses problems of an increased amount of scale spalling and of an expensive cost of the material.
U.S. Pat. No. 4,640,722 discloses and claims a ferritic stainless steel exhibiting improved cyclic oxidation resistance and creep strength, after a final anneal at 1010.degree. to 1120.degree. C., which develops Nb-Si rich Laves phases, suitable for use in the manufacture of automobile exhaust gas systems, consisting essentially of, in weight percent, C.ltoreq.0.05%, Mn.ltoreq.2%, 1.0% <Si.ltoreq.2.25%, Al<0.5%, 3.times.Al.ltoreq.Si, 6%.ltoreq.Cr.ltoreq.25%, Mo.ltoreq.5%, 8%.ltoreq.Mo+Cr, N.ltoreq.0.05% 4.times.C+3.5.times.N.ltoreq.at least one of Ti, Zr and Ta.ltoreq.0.5%, total Nb.ltoreq.0.3%, 0.1%.ltoreq.uncombined Nb, and balance essentially Fe. However, this U.S. Patent does not teach how to suppress oxide film spalling nor address to low temperature toughness and fabricability of the steel. For use in the manufacture of automobile exhaust manifold, the steel is required to have improved adhesion of oxide film and low temperature toughness in addition to impr
REFERENCES:
patent: 4417921 (1983-11-01), Maurer
patent: 4461811 (1984-07-01), Borneman et al.
patent: 4640722 (1987-02-01), Gorman
Hiramatsu Naoto
Oku Manabu
Uematsu Yoshihiro
Nisshin Steel Co. Ltd.
Yee Deborah
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